First, thanks for the kind words, guys.
Next, most plasma drivers do use helium AFAIK, and the ozone levels generated are very small. That was one of the things Dr. Hill addressed and it has been raised by numerous other designers. They are mostly horn-loaded IIRC and the plasma driver's acoustic output is actually pretty small. That said, I have done essentially *zero* research on plasma drivers in the last decade or three. The cost of helium tanks and recharging once or twice a year is a major PITA for many folk, and heat is definitely an issue for most of the designs. Integrating the plasma driver(s) to bass drivers (essentially required to get enough LF output) is also a major challenge.
One cool thing about plasma drivers is that, as you push the volume to the limits, the gas simply stops expanding and sort of self-limits in a "soft" way. I still remember watching and listening as the Hill speakers were overdriven and noticed how the plasma driver just sort of softened out without getting all harsh or cracking mechanically like cones or panels. Or maybe I've just forgotten after so many years...
There have been a few other very unconventional designs over the years that would be worth a search (not sure when I'll have time -- few other things going on right now). One I recall used ultrasonic vibrations from an array of piezo drivers or some such and a modulator to allow it to "place" the coherent sound at a particular point in space. Out of that spot, you heard some noise/hiss or nothing. Audio beamforming. There was another trying to use a laser to manipulate... something, can't recall what... and create "holographic" sound. I only read about that one, never saw (heard) a demo, and never heard anything else about it. There was a group at one of the research labs toying with the idea of plasma drivers and superconducting containment fields. My contract was on something else but they were piddling in the back room with it and led me through it. The base application was military but they thought the (new then) "high-temperature" superconductors (using liquid nitrogen instead of liquid helium) might make it practical for consumers. Again I never heard any more about it. Ditto ideas for using a plasma "sheet" instead of a sphere, sort of a plasma ESL panel. Containment and alignment/control of the plasma sheet was pretty challenging IIRC.
Now having said I've no time to actually do the work, I think a (maybe just this) thread on these and other very unconventional designs would be interesting. I imagine many folk on ASR have much more and deeper knowledge than I; I really am not a speaker designer, haven't really done any since my DIY servo sub a long, long time ago, and my day job is far removed from audio. For now I get by with my Revels, when I have time to listen!
I suspect a few relevant posts from our Central Scrutinizer (
@RayDunzl) are forthcoming...
